see also:

• bushfires in Australia
• major flooding events in Victoria
• Melbourne weather
• Melbourne weather records - heat waves, floods, tsunamis and other extreme events

• regular weather events are:
  • seasonal tropical cyclones
  • Low pressure systems and troughs which can be slow moving and bring much rain and wind storms
  • High pressure systems can bring extreme hot days and bushfires
    • in a normal Summer these subtropical ridges tend to be centred over southern NSW/Victoria latitudes
      • in a positive SAM these can migrate south to over Tasmania bringing strong easterlies to southern Victoria
      • Summer/Autumn tropical Lows migrating south can also push a High southwards over Tasmania
    • in a normal Winter these tend to be centred over mid NSW / southern Qld latitudes
  • Cold fronts particularly effect southern parts of Australia and bring storms, flash flooding, cold winds and showers
    • these originate from circum-polar systems and the extent they extend over southern Australia depends upon SAM and the position of the subtropical ridges of High pressure

• The Indian Ocean Dipole (IOD)
  • defined by the difference in sea surface temperature between two areas (or poles, hence a dipole) – a western pole in the Arabian Sea (western Indian Ocean) and an eastern pole in the eastern Indian Ocean south of Indonesia.
  • The IOD usually collapses as the monsoon moves down at the beginning of Australia's summer
  • a positive IOD results in:
    • warmer sea surface in western parts of Indian Ocean
    • less cloudiness over NW of WA
    • less rainfall over southern Australia and the North End
  • a negative IOD results in:
    • warmer sea surface in eastern parts of Indian Ocean
    • winds become more westerly bringing increased cloudiness to NW of WA
    • more rainfall over southern Australia and the North End as Low pressure systems from NW Australia move to Victoria dragging with it higher than normal levels of tropical moisture in Spring
• Madden-Julian Oscillation
  • brings monsoonal troughs and tropical rain over northern Australia from the Indian Ocean in summer
  • moves eastwards at about five metres per second, so spending around five days in each location as it moves over and it often leads to several days of enhanced lift, cloudiness, storms and rainfallit can also have enhanced rainfall in the southern part of Australia, although this is a much lesser impact
  • when active over northern Australia, it often leads to several days to weeks of enhanced rainfall and flooding.
  • before and after the MJO is an area of hot, dry conditions due to downward airflows radiating out from the upward airflows in the MJO region and thus an MJO can block or enhance the effects of the La Niña in the western Pacific and it can kickstart the formation of an El Niño
  • usually takes between 30 and 60 days for the MJO to move from where it forms in the Indian Ocean, across the Maritime Continent (a term meteorologists use to describe Indonesia, the Philippines and Papua New Guinea) and the Pacific before it dies out over South America
• The Southern Oscillation Index (SOI or ENSO)
  • a measure of the intensity or strength of the Walker Circulation over the Pacific Ocean near the equator
  • measures the difference in surface air pressure between Tahiti and Darwin
  • Sustained positive SOI values above about +8 indicate a La Niña event due o cooler ocean near America while sustained negative values below about –8 indicate an El Niño due to a warmer ocean near America
  • La Niña events result in:
    • stronger easterly trade winds near the equator
    • hotter drier conditions in central America
    • increased tropical cyclone activity and increased rain, floods and clouds to NE Australia in partucular, but also down the east coast to Gippsland as the tropical Lows travel south down the coast
    • overall increased cloudiness over Australia lowering day time temperatures and raising night time minimums
    • southern parts of Australia, particularly SW of WA and to a lesser extent, southern Tasmania may have drier than normal conditions
    • increased frequency of Summer positive SAM events and increased rain to SE Australia
    • La Nina climatic conditions could become the norm if the Atlantic Overturning Meridional Circulation slows down or ceases ¹⁾
    • triple La Nina events in consecutive years is relatively rare - 1954-57, 1973-76 and 1998-2001 and we might be due for one in late 2022
  • El Niño events result in:
    • increased clouds, rain over central America
    • drier, hotter conditions in Australia, especially the eastern half of Australia in Winter/Spring
• The Southern Annular Mode (SAM)
  • http://www.bom.gov.au/climate/sam/images/SAM-in-Australia.pdf
  • also known as the Antarctic Oscillation, describes the north–south movement of the westerly wind belt that circles Antarctica, dominating the middle to higher latitudes of the southern hemisphere.
  • Each positive, neutral or negative SAM event tends to last for around one to two weeks or a few months, and occur weeks to months apart
  • positive phase results in:
    • the band of rain bearing westerly winds contracting towards Antarctica
    • higher pressures over southern Australia
    • more stable, dry conditions over central and western parts of southern Australia
    • in Summer, the more southerly Highs direct moist easterlies onto the SE coast on Australia resulting in increased rains to SW Aust
    • was a significant contributor to the 'big dry' observed in southern Australia from 1997 to 2010 and a positve SAM from Mar 2010-Feb 2011 may have contributed to SW WA not getting the rains form the 2020-2012 La Nina events despite a positive IOD
    • during autumn, a positive SAM typically has a weaker influence on Australian rainfall, but may have a slight drying influence on the far south-west of Western Australia.
  • negative phase results in:
    • Band of westerly winds expands towards the equator
    • More (or stronger) low pressure systems over southern Australia
    • In Winter:
      • increased storms and rain to SW WA and to Tas/Vic and drier over NSW
    • In Summer:
      • drier conditions over most of southern Australia (except for west coast of Tas) as it allows more central Aust winds to flow to SE Australia
• ocean water temperatures around Australia
  • warmer than usual ocean temperatures generally increase the amount of rain and may even offset an El Nino
• high altitude jet streams
  • these form where the cold air of the poles meets the relatively warm air of the equator and the wind speed increases with temperature differentials and can reach 400kph in Australia's winter
  • two main jet streams:
    • subtropical jet streams (at latitudes 30-32deg S but only in cooler months in Australia as they dissipate in summer)
      • the further from the equator, the less rain-bearing systems it helps carry
    • polar jet streams (over Southern Ocean)
  • these are circumferential around earth in a sine wave like pattern and can steer weather systems such as cold fronts and lows in a west to east direction and can increase the strength of Low pressure systems bringing floods to southern Australia, and if they stall or split into two can block High systems moving causing either heat waves or floods - blocking behaviour is more common in Nth Hemisphere due to greater land mass and thus greater temperature differentials, but also lower temp differentials due to global warming tends to make them weaker and more wavy resulting in extreme weather events)

• the main factors affecting snow resort depth of snow in winter are (with average affect on snow depths):
  • El Niño event reduces snow depth by 17%
  • positive Indian Ocean Dipole (IOD) event reduces snow depth by 15%
  • El Niño event PLUS a positive Indian Ocean Dipole (IOD) event reduces snow depth by 23%
    • However, this results in increased clear skies at night meaning frosty nights which are good conditions for snowmaking machines
  • negative Indian Ocean Dipole (IOD) event INCREASES snow depth by 20%
  • La Niña has variable effect as the average 25% increased rain in SE Australia may not fall as snow and so snow depth may only increase by as little as 4%
  • a neutral Indian Ocean Dipole (IOD) season with a neutral Pacific Ocean (no El Nino or La Nina) generally gives 8cm better than average snow depths
  • the frequency of cold fronts, however these cannot be accurately predicted more than a few weeks ahead
  • climate change which is slowly reducing snow seasons in Australia

• El Nino event late 2023 - early 2024
• a positive Southern Annular Mode (SAM) - perhaps due to a longer lasting ozone hole and higher sea ice levels) has been the key driver in bring unusually wet weather to SE Australia with the westerly belt of winds being closer to Antarctica drawing tropical, warm, moist air further south and allows for more onshore winds over the east coast, which again helps push moist air
• Dec 2023-Jan 2024, 7th Jan: heavy rain and flooding events in central Victoria in particular results in evacuations of Seymour and Yea plus Rochester and significant flooding to other areas such as Bendigo, Heathcote
• Feb 13th 2024: Squall line of severe thunderstorms cause bushfires in Grampians and 120kph wind gusts wreaking havoc especially in Mirboo Nth but also knocking down power transmission lines near Geelong causing widespread power outage
• Feb 18th 2024: Geraldton record breaking 49.3degC - hottest day of record for Geraldton
• Perth has driest summer on record with only 21.8mm rain from Oct 23-end of March 2024 (prior record was 37mm in 1994-95) - WA relies heavily on groundwater stored in aquifers, with Perth's main freshwater source being the Gnangara Mound and this is drying up - most of Perth' rainfall tends to be in winter - similar to Mediterranean regions
• Melbourne has a very dry March with only 2.8mm rain (this was followed by 60-70mm on 1st April due to low pressure system from ex-cyclone)

• warmer than usual ocean temperatures cause increased rain events in early months of 2023
• warmest winter on record
• 2023 was the 8th warmest on record for Australia
• 2023 was also characterised by increase in high rainfall bursts - for every 1degC increase in global warming it is thought that 1hr downpours will generate 15% more rainfall and 24hour events will give 8% more rainfall and storms are becoming more intense with stronger wind gusts - the warmer the sea surface, the more energy for storms and frontal systems.
• Aug-Oct 2023 was Australia's driest 3 month period on record which was then followed by 3 months of above average rainfall (except for Tas and much of WA which had record dry periods in parts)
• Sept: driest and warmest Sept on record for Victoria, many sites had their highest September temperature on record, leads to early major bushfires in Gippsland followed by floods
  • Mildura Airport recorded 20 days of maximum temperatures 20.0 °C or above - previous record was 13 days in 2013
• Positive Indian Ocean Dipole event appears to be beginning July 2023 - the 1st since 2019 - and likely to result in very low winter-spring rainfall and clear skies, high daytime temperatures for Vic and central Australia in particular - these usually peak from August to October, then rapidly decays when the monsoon arrives in the Southern Hemisphere in November or December
• late Dec: heavy rain and flooding events in east coast of Australia

• 2nd La Nina in a row brings major flooding to east coast of Australia and another mild Summer to Victoria
  • March 2022: flooding particularly impacts Lismore northern NSW with two separate floods
• negative Indian Ocean Dipole declared Aug 2022
• 3rd consecutive La Nina event declared Sept 2022
• Oct 2022: record flooding in eastern Australia as cold fronts pushed southwards to southern Tas which allows moisture filled Low pressure systems thanks to neg IOD and the La Nina from northern WA to slowly drift eastwards

• major wind storms devastate much of Victoria's alpine forests
  • 29th Oct 2021: Covid-19 lockdown ends and a strong Low pressure system brings 122kph gusts to parts of Melbourne and 143kph gusts to Wilsons Prom and Mt William in the Grampians. Some stations in SE Melbourne recorded their strongest gusts on record.
  • early June 2021 low pressure system produces over 200mm rainfalls and 50cm snow recorded in Vic alps flooding much of Gippsland especially Traralgon while the strong winds knocked over thousands of tall gums in the Dandenongs destroying over 120 houses and damaging a further 100-200 as well as knocking out the power grid to over 5000 homes for 4 weeks and potable water had to be trucked in. Severe storm damage especially impacted Latrobe and Baw Baw Shire with popular spots around Licola, Bruntons Bridge, Coopers Creek, Donnellys, Tarra Bulga and the Strzeleckis closed to campers for some time.
  • late Jan 2021, strong easterlies bring down thousands of trees in Wilsons Prom due to Low pressure systems off the NSW coast pushing Highs south-eastwards over Tasmania and creating persistent strong dry easterlies.
• extreme flooding to much of NSW in March 2021 tracks down to Gippsland and Wilsons Prom
  • 23rd March 2021:
    • a blocking High pressure system in the Tasman Sea creating a Low trough in eastern coast of NSW causing extreme flooding and then developed into a blocked low which travelled south to a few kms east of the Prom resulting in over 100mm rain in 2hrs at lighthouse with winds next evening gusting over 130kph. In 2 days, Tidal River recorded some 230mm rain and lost perhaps 6-10“ of beach sand and the creeks changed their route to the sea. The Park was closed for 2 days.
• La Nina event of summer 2020-2021 - brings coldest summer in 19yrs for Melbourne

• South-East Australia bushfires of Black Summer 2019-2020
  • Jan bushfire smoke dominate Canberra, Melbourne and eastern Victoria from fires in Tasmania and in Gippsland making for an uncomfortable summer
    • consuming much of Australia's supplies of face masks which would be much needed in March 2020 for the start of the Covid-19 pandemic
  • over 10 million hectares burnt across Australia form July 2019-Jan 2020 and 33 people killed
  • over 1 million hectares burnt in eastern Victoria
  • much of Kangaroo Island in Sth Australia burnt
  • 36,000 ha burned in Fingal region Tasmania
  • see also:
    • BBC coverage of Australian bushfires and warming

• Dec 30th 2019:
  • Almost all of Tasmania (TAS) recorded accumulated monthly forest fire danger indices (FFDI) in the highest 10 per cent of historical values for December, and much of the eastern half of the state recorded its highest-ever December FFDI on 30 December. Several locations recorded temperatures in the high 30s and low 40s that day, with several experiencing a temperature record for December. TAS registered 406 lightning strikes that ignited dozens of bushfires that day, which would result in 36,000 ha burned in TAS.
• Dec 2019 - hottest December day on record for Melbourne;
• Dec 2019: Record warmth in Australia during December 2019 was accompanied by record low rainfall over eastern Australia, and followed very much warmer than average and drier than average conditions across much of Australia through most of the year.
• “Black Summer” Nov 2019-Jan 2020, major bushfires burn much of eastern Victoria (east of Bairnsdale) and the east coast of NSW
• Australia drier than usual due to a Positive Indian Dipole

• western Victoria and the Otways suffer heavy rains resulting in prolonged closure of the Great Ocean Rd due to landslides and rockfalls
• 27th Jan 2016, 1 in 100 yr rain storm delivers 40-70mm rain in 1hr causing flash flooding in Geelong, Vic

• 8-9th Dec 2015, over 80mm rain in 24hrs brings flooding to Ovens River area in NE Vic

• March 2012 NSW/Victoria flood event:
  • the flooding event of March 2012 saw 75 per cent of New South Wales under flood warnings, forcing evacuations and natural disaster declarations. It followed record rainfall that also affected northern Victoria where communities were evacuated.
  • Rainfall starting in late February fed into river systems, causing trouble for communities stretching south in a line from Broken Hill to Sydney.
  • Wagga Wagga, Forbes were affected
  • Sydney experienced flash flooding as torrential rain fell and western Sydney recorded flooding for the second month in a row as the Warragamba Dam reached full capacity and began to spill.
  • in NSW north west, major flooding was recorded at communities around Bourke as the Darling River began to swell, taking water from rainfall across NSW and Queensland from earlier in the year.
  • NSW statewide average rainfall of 119.59mm was more than double the average, making 2012 the state's second wettest March on record and the wettest March since 1956.
  • in northern Vic, heavy rainfall saw floodwaters move across the region and cut access to Nathalia, Numurkah and surrounding communities.

• western and central Victoria in particular affected by flooding more severe than in 2010 due to one of the strongest La Nina events since the late 1800s
• flooding in western Victoria during the 2010–11 La Niña event originated from two tropical cyclones (Anthony and Yasi) which decayed over central Australia before moving south.
• Averaged across Australia, 2010 and 2011 were the second and third wettest years on record, with only 1974 trumping them. If we then include the February/March 2012 event, the 24 month period from April 2010 to March 2012 was Australia’s wettest 2 year period in recorded history.
• March 2011:
  • Wilsons Prom in Victoria major flooding event:
    • Tidal River camp ground was flooded (up to one metre of water inside the park’s information centre) and the bridge over Darby River was cut stranding 300 campers and requiring helicopter transport out (including 53 picked up from Sealers Cove) after a freakish storm (thought yo be a 1 in 3000 year event) delivered 370mm rain in 24hrs (Victoria's record is 375mm in 24hrs in the Otways in 1983, next highest is 318mm at Mt Wellington in 2007 due to an east coast low). A large land slip also resulted visible from the Lilli Pilli Circut Walk in addition to land slips on the Telegraph Road which was also lost in two sections. Loo-Ern track was destroyed and the cabins had to be re-built. Debris from flooding was on top of the railing of the pedestrian bridge at Tidal River. Tidal River and areas south of Cotters Beach were closed to public for months.
• February 2011 thunderstorms across Victoria led to flash flooding around Melbourne and other isolated areas, with a tremendous downpour on Mildura recording almost 270 mm of rain over 36 hours, with 145 mm in just 2 hours.
• January 2011 most of Victoria was drenched with record rains.
  • a persistent low pressure system and this tropical moisture resulted in over 150 rainfall stations across Victoria recording their wettest January ever, as well as their highest January daily rainfall.
  • Many rivers experienced their highest recorded flood levels, some by a long way, with the event comparable to the historic 1909 flood on the northern rivers.
  • The north-west and central regions were hardest hit with the Avoca, Campaspe, Loddon and Wimmera river systems all experiencing one of the largest floods in living memory. The floods took a long time to wash down through the system, with the lower Loddon River taking months to drain downstream of Kerang in the Benjeroop area.

• increased rainfall bring major floods and an end to a record breaking prolonged drought in SE Aust
• December 2010 again saw Major flood levels for rivers in the north-east, and severe thunderstorms resulted in significant damage over the eastern suburbs of Melbourne.
• November 2010 brought Major flooding to the Loddon and Avoca Rivers, with flash flooding over the western suburbs of Melbourne.
• October 2010 again saw rivers in Victoria reach Major flood levels in the north-east on the Upper Murray River.
• 3-5th Sept 2020:
  • In Victoria every river basin north of the Great Dividing Range experienced Major flooding except the Campaspe River which recorded Moderate flooding.
  • The September 2010 flood event resulted in widespread riverine flooding and flash flooding across south-east Australia, devastating many communities.
• August 2010 the first of the heavy rain began, with a State wide monthly average over 100 mm, the first time since 1993.

• The 2000s drought in Australia, also known as the Millennium drought is said by some to be the worst drought recorded in Australia since European settlement
• This drought affected most of southern Australia, including its largest cities and largest agricultural region (the Murray–Darling basin). It commenced with low rainfall conditions in late 1996 and through 1997, and worsened through particularly dry years in 2001 and 2002. By 2003 it was recognised as the worst drought on record.
• The year 2006 was the driest on record for many parts of the country and conditions remained hot and dry through to early 2010.
• The drought placed extreme pressure on agricultural production and urban water supply in much of southern Australia.
• It led to the construction of six major seawater desalination plants to provide water to Australia's major cities, and to changes in the management of water in the Murray–Darling basin, particularly the formation of the Murray–Darling Basin Authority.
• It culminated on 7-8th Feb 2009 with Victoria's devastating Black Saturday bushfires
  • followed Victoria's 15th lowest annual rainfall on record of 504mm in 2008 and 12 years drought
  • the fires occurred after a record 3 days straight of temperatures above 43degC.
  • Victoria suffered its most ferocious and tragic bushfires which suddenly roared to life after midday on Melbourne's hottest day on record when temperatures reached 46.4 degC in the CBD and 47.9deg C at nearby Avalon, while winds of 90-100kph fanned the flames forcing fires to traverse 1.5km in 5min leaving last minute escape impossible for many.
  • over 2000 homes were lost, 173 people died and millions of animals were killed.
  • Marysville, Kinglake, and Strathewen townships were devastated.